Phased array antennas are known in the art to be well suited for communication applications, which require substantial gain, multiple agile beams, and broad surface area coverage, for example, on satellites in mid-earth or geosynchronous orbits. The diameter of the earth as viewed from a geosynchronous satellite subtends a satellite field of view of approximately only .+-.8.5 degrees. In addition, it is well known that phased array antennas have terrestrial applications.
Phased array antennas typically include a plurality of radiative elements arranged in a two-dimensional pattern. To decrease the number of radiative elements, and therefore the costs of building a phased array antenna, radiative elements are often spaced as far apart from one another as possible within the design specifications of the antenna. Radiative elements for antennas which are to be utilized on satellites in mid-earth or geosynchronous orbits can be separated much further than radiative elements for antennas to be utilized on satellites in low earth orbits.
Separating radiative elements beyond the wavelength .lambda. of the transmitted or received signals results in the formation of grating lobes (i.e., beams that form in directions other than the direction of interest). Grating lobes result in a reduction of antenna gain in both transmit and receive modes. Accordingly, it is generally preferable to eliminate or reduce the power radiated into grating lobes.
Typically, grating lobes are eliminated or diminished by using smaller radiative elements which are spaced closer together. Natural zeros, or nulls, in the radiation pattern occur at angles between the main lobe and the grating lobes. Accordingly, the aperture of the element is typically designed to control the location of the natural zeros.
One approach to designing antenna element apertures is based on achieving an extended aperture dimension by creating "overlapping subarrays" which utilize interconnecting networks feeding the array elements. These interconnecting networks add significant complexity to the beam-forming process and, consequently, have had very limited practical application.
It is well known to those skilled in the art that under certain conditions phased arrays are subject to an anomalous null, which exists inside the natural zero, a phenomenon known as "blindness". One example of the blindness phenomenon is described in detail in a paper by Oliner, Arthur A., "Surface Wave Effects and Blindness in Phased Array Antennas", from Phased Array Antennas, ARTECH House, 1972, pp. 107-112.
Another example of a type of "blindness" applicable to arrays with large element spacing (i.e. greater than one wavelength) is referred to herein as "forced modal resonance" and is described in the Amitay and Gans paper, "Design of Rectangular Horn Arrays with Oversized Aperture Elements", IEEE Transactions on Antennas and Propagation, Vol. AP-29, No. 6, pp. 871-884 (November 1981).
The blindness phenomenon is typically manifested by the existence of deep "anomalous" nulls in the embedded element pattern. If the array is large and the element pattern is for an interior element, then these nulls appear symmetrically disposed. Edge elements demonstrate the nulls asymmetrically. The existence of anomalous nulls in the embedded element pattern means that if the array antenna is phased to point a beam in those directions, total reflection will occur. Heretofore, complex and costly techniques have been developed to eliminate or reduce the effect of anomalous nulls within the antenna's FOV requiring additional hardware and software.
Conventional phased array antennas are seldom proposed as antennas for high-gain, limited-scan applications because the required element spacing is small, and the resulting number of elements and phase shifters is excessively large. It has long been recognized, however, that if flat-topped radiation patterns could be synthesized to suppress the grating lobes, arrays of relatively few but larger sub-arrays or elements could be used for these applications.
Accordingly, a need exists for extending the effective aperture of the radiative elements of a phased array antenna without incurring additional complexity and cost to overcome the blindness phenomenon.